The present invention relates to a liquid crystal display device and, more particularly, to a so-called active matrix type of liquid crystal display device.
The active matrix type of liquid crystal display device includes a pair of transparent substrates disposed in opposition to each other with a liquid crystal interposed therebetween, gate signal lines formed to be extended in an x-direction and to be juxtaposed in an y-direction on a liquid-crystal-side surface of either one of the pair of transparent substrates, drain signal lines insulated from the gate signal lines and formed to be extended in the y-direction and to be juxtaposed in the x-direction on the liquid-crystal-side surface, thin film transistors which are respectively formed in areas each surrounded by these gate and drain signal lines and are driven by supply of scanning signals from the gate signal lines, and pixel electrodes to which video signals from the drain signal lines are to be supplied via the respective thin film transistors.
These gate signal lines and drain signal lines are formed by micromachining using so-called photolithographic techniques. Accordingly, in the process of manfacturing the liquid crystal display device, it is common practice to inspect whether these gate and drain signal lines are formed without disconnection or whether there is a short circuit between each of these signal lines.
In this case, in order to enable such an inspection to be performed efficiently, a short-circuit lines to which each of the gate signal lines is connected at one end in common and a short-circuit line to which each of the drain signal lines is connected at one end in common are formed, although these short-circuit lines are separated from the gate and drain signal lines by cutting in a later process.
For this reason, on the transparent substrate of the liquid crystal display device manufactured as a product, there remain traces which make it possible to infer that the gate signal lines and the drain signal lines were previously connected to the corresponding short-circuit lines.
The thin film transistor formed in each pixel area is an MIS type transistor which includes a gate electrode made of a portion of the corresponding one of the gate signal lines and a drain electrode made of a portion extended from the corresponding one of the drain signal lines. Accordingly, measures for preventing those signal lines from being charged with concentrated static electricity are taken in order to prevent variations in the threshold voltage of the thin film transistor or breakdown thereof (refer to, for example, Japanese Patent Laid-Open No. 27263/1993).
Specifically, a common line for electrostatic protection which is connected to all of the gate and drain signal lines via nonlinear resistance elements is formed at the outer periphery of a display area formed of an aggregation of the pixel areas, so that static electricity produced in part of the signal lines is dispersed into all of them by the common line. (A circuit constructed in this manner will be hereinafter referred to also as an electrostatic protection circuit.)
However, in the liquid crystal display device constructed in this manner, it has been discovered that the threshold voltages (Vth) of the thin film transistors in the respective pixel areas show dispersion after the completion of the manufacture of the liquid crystal display device.
There are various causes for the variations of the threshold voltages of the thin film transistors, and it may be considered that a method of eliminating such causes is adopted as one method. However, a method of applying predetermined processing to a thin film transistor whose threshold voltage has shown dispersion and uniformizing the threshold voltage is preferable in terms of the efficiency of manufacture.
The present invention has been made in view of the above-described circumstances, and aims to provide a liquid crystal display device which is capable of readily achieving uniformization of the threshold voltage of each thin film transistor in spite of its simple construction.
Representative aspects of the present invention disclosed herein will be described below in brief.
The liquid crystal display device according to the present invention basically includes gate signal lines juxtaposed in one direction, drain signal lines juxtaposed to intersect with the gate signal lines, thin film transistors which are respectively formed in areas each surrounded by the gate signal lines and the drain signal lines and are driven by supply of scanning signals from the gate signal lines, pixel electrodes to which video signals from the drain signal lines are to be supplied via the respective thin film transistors, a first terminal for applying voltages to the respective gate signal lines via elements each of which allows a current to flow in one direction, and a second terminal for applying voltages to the respective drain signal lines via elements each of which allows a current to flow in one direction.
In the liquid crystal display device constructed in this manner, after the manufacture of the thin film transistors, by applying a voltage between the first terminal and the second terminal, a predetermined voltage can be applied between the gate and the drain of the thin film transistor in each pixel area for a predetermined time.
Accordingly, it is possible to stabilize the structure of each of the thin film transistors that contains a channel layer, thereby uniformizing the threshold voltages of the thin film transistors.
A circuit which connects the first terminal to each of the gate signal fines via the elements and a circuit which connects the second terminal to each of the drain signal lines via the elements remain even after the liquid crystal display device has been manufactured as a product.
Even if such circuits remain, the gate signal lines or the drain signal lines can operate as lines independent of (insulated from) one another during the supply of signals owing to the elements each of which allows a current to flow in one direction.